1. Field of Invention
The present invention relates to a low maintenance high pressure viscometer.
2. Description of Prior Art
In connection with the drilling of oil and gas wells, drilling fluid is commonly used to drive drill bit and bring sand and stone cuttings back to ground surface. Viscosity property of drilling fluid is critical in the drilling process. A drilling fluid with excessive viscosity would make it difficult to pump it down to bore hole, while a drilling fluid with insufficient viscosity would make it difficult to carry sand and stone cuttings back to ground surface. The viscosity property of a drilling fluid varies significantly with the change of temperature and pressure. Thus a viscometer capable of closely simulating down-hole conditions with low maintenance is of great interest. Down-hole conditions are typically from room temperature and pressure up to 40,000 psi and 600° F.
A few types of arrangements have been applied to measure the viscosity of liquids under high temperature and high pressure conditions. In U.S. Pat. No. 3,435,666, a helical spring is attached to the inside bob through a bob shaft while driving the outer cylinder. The shear force applied on the bob is proportional to the torque applied by the liquid, which is also measured by a strain gauge torque transducer. One of the drawbacks of this design is that packing 41 is required to dynamically seal the rotating tube 32. Due to the nature of the dynamical seal, it has difficulty to seal above 2,000 psi. Thus any test conditions above 2,000 psi in pressure will be difficult to achieve. In U.S. Pat. No. 4,633,708, a sealed container within a high pressure vessel is used for testing rheologically evolutive materials. One of the drawbacks of this design is that seal 12 is used to separate the test sample from outside pressurizing fluid. Measurement error due to the friction between seal 12 and shaft 14 is inevitably added. For very thick test samples such as cement this error is possibly tolerable. However, a typical drilling fluid under high temperature and pressure conditions has a typical viscosity of 10 cP to 30 cP when shear rate is around 500 l/s. This friction-induced error is too high to provide meaningful results. In U.S. Pat. No. 4,466,276, an open top slurry cup within a high pressure vessel is used to measure cement consistency. One of the drawbacks of this design is that due to the wide-open top of the slurry cup, the sample can mix with pressurization fluid easily, which leads to inaccurate results. In model 75 viscometer manufactured by Fann Instrument Company, a pair of v-shape jewel bearings is used to support a complicated bob assembly whose rotational movement is restricted by a helical spring. One of the drawbacks of this design is that the jewel bearings are fragile, easy to break, prone to wear and expensive to replace. Another drawback of this design is that helical spring assembly is generally much more complicated and spacious comparing to spiral spring assembly.
It is an object of this invention to provide a high pressure viscometer wherein viscosity is determined under conditions closely simulating down-hole conditions.
It is another object of this invention to provide a high pressure viscometer that eliminates measurement errors due to seal frictions.
It is another object of this invention to provide a viscometer that requires substantially less maintenance work yet meets industry standards of accuracy, reliability, durability, dependability, and ease of cleaning.